The manufacture of semiconductor devices comprises the composite of numerous processing steps on each semiconductor device. A variety of attempts have been made to establish quality control checks on the various processes during the manufacture of the semiconductor devices. The systems for these quality control checks usually include some manner of physically inspecting or testing selected specimens of the semiconductor devices at various stages during the manufacturing processes. However, such systems are often cumbersome, relatively costly, and since they are conducted, in many cases, only on randomly selected devices from the manufacturing processes, cannot guarantee the quality of each of the devices manufactured. Moreover, the previously developed systems serve to detect problems in the manufacturing process at points in time after the errors have already been made. As a consequence, the same error may have been repeatedly made during a particular processing operation on a large number of semiconductor devices before the error is detected at a later quality control inspection. It is clearly of economic interest to the manufacturing concern to detect aberrations in the manufacturing processes as early as possible after those aberrations occur. It is even more preferable to be able to detect those aberrations in real time, in other words, at the point in time at which they are occurring. Such instantaneous detection can be used to avoid duplication of the same process aberration to repeated semiconductor devices during manufacture.
A common processing operation in the production of semiconductor devices is a plasma etch. In the plasma etch, a semiconductor device in a form commonly termed a slice is positioned in an etching chamber in the presence of specified gases, at predetermined pressures and temperatures, and an RF power source is applied. In the etch, particular materials on the surface of the semiconductor slice react with gases in the chamber and then volatilize from the surface of the slice. A typical etch process for a slice can proceed for less than one minute up to or over several minutes.
Conventional etch reactors typically include apparatus to monitor particular aspects of the etching process. The reactors include apparatus to provide on-line hardware monitoring, e.g. to monitor the temperature and pressure of the reactor, the wattage of the RF power source, and the flow of feed gases. Prior art etching equipment also includes apparatus designed to detect the end-point of the etching operation. Methods of End-Point Detection for Plasma Etching; Paul J. Marcoux and Pang Dow Foo, Solid State Technology, April 1981, pp. 115-122, incorporated herein by reference, describes several methods proposed for such apparatus. Such proposed methods include methods of emission spectroscopy, optical reflection, mass spectroscopy, impedance monitoring, Langmuir probe monitoring and pressure monitoring. The end-point monitoring apparatus, with whatever monitoring method is used, serves to detect the end of the desired etch reaction so that the etcher can be instructed to terminate its etch cycle and ready itself to etch a fresh slice.
A frequently applied method of end-point detection utilizes an end-point trace (EPT). An EPT is a measure, obtained by emission spectroscopy procedures, of the concentration of gases in the plasma over the surface of the slice being etched. The EPT can be designed to monitor either reactants or products of the etch reaction. For example, by tuning the EPT to measure those gases which are desired etch products, the monitoring apparatus can detect when those products are no longer being emitted into the plasma, thus signaling the end of the desired etch reaction. Typically, the end-point detectors are designed to look for a sharp change in the concentration of the monitored species at a time into the etch process approximately when the end-point is expected. The apparatus is useful in that it provides a signal to the etching equipment to proceed to end the etch cycle, remove the etched semiconductor slice, and insert a fresh semiconductor slice into the etching chamber.
The end-point detectors, whether applied or merely theoretical, however, are subject to several limitations. A principal limitation is that the detectors serve only to detect the end of etching operations. Accordingly, the detectors provide no information as to whether the etch process has proceeded in an optimum fashion or whether aberrations in the process occurred.
A need has arisen for a process or apparatus which can determine whether the etching process is proceeding or has been accomplished in an optimum fashion. Such an etch monitoring system which could automatically check the etch of every slice would also overcome numerous disadvantages referred to above with regard to current semiconductor manufacture quality control systems. Moreover, it would be advantageous if a system could monitor the etch process in such a fashion as to provide information regarding whether layers formed on the semiconductor device prior to the etching operation were formed and treated in the intended manner.
The prior art devices and processes are not able to provide the above mentioned desired advantages.
In a co-pending application Ser. No. 046,497,currently pending, assigned to the assignee of the present application. and filed May 4, 1987, before the invention of the present application, is disclosed a method and apparatus for detecting aberrations in cyclically repeated process operations. A particular application of the invention described in that application is in regard to plasma etch processes in the production of semiconductor devices. In an embodiment of the invention of that application, the actual end-point trace (EPT), for the etch of each semiconductor slice is compared in detail with a predetermined reference EPT to determine not only whether the etch has proceeded as intended, but also to determine whether certain operations prior to etching have been properly carried out on the slice.
The present invention provides an improved method and apparatus to that disclosed in the related application.